Rust inhibitor for gasket, gasket for secondary battery including same, and secondary battery including gasket for secondary battery

ABSTRACT

A rust inhibitor for a gasket, a gasket for a secondary battery including the same, and a secondary battery are provided. The rust inhibitor includes an anti-rust material and a polymer resin, wherein the polymer resin includes at least one of polyethylene or a copolymer which includes 2 or more derived units selected from the group consisting of an ethylene-derived unit, a propylene-derived unit, a butylene terephthalate-derived unit, an ethylene terephthalate-derived unit, and a methyl acrylate-derived unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national phase entry under 35 U.S.C. § 371of International Application No. PCT/KR2019/002284 filed Feb. 25, 2019,which claims the benefit of Korean Patent Application No.10-2018-0022064, filed on Feb. 23, 2018, and Korean Patent ApplicationNo. 10-2019-0021675, filed on Feb. 25, 2019, in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein in itsentirety by reference.

TECHNICAL FIELD

The present invention relates to a rust inhibitor for a gasket, a gasketfor a secondary battery including the same, and a secondary batteryincluding the gasket for a secondary battery.

BACKGROUND ART

In general, unlike a primary battery which is not chargeable, asecondary battery refers to a battery chargeable and dischargeable, andis widely used as a power source of electronic devices such as a cellphone, a notebook computer, and a camcorder, or of an electric vehicleand the like. Particularly, a lithium secondary battery has a drivingvoltage of 3.6 V, and has a capacity of approximately three times morethan a nickel-cadmium battery or a nickel-hydrogen battery, which arewidely used as a power source of electronic equipment. Since a lithiumsecondary battery has high energy density per unit weight, theutilization thereof is rapidly increasing.

The lithium secondary battery usually uses a lithium-based oxide and acarbon material as a positive electrode active material and a negativeelectrode active material, respectively. In addition, a lithiumsecondary battery may be classified into a square-shaped battery, acylindrical battery, and a pouch-shaped battery.

A lithium ion secondary battery is provided with an electrode assemblyin which a positive electrode/a separator/a negative electrode aresequentially disposed and an exterior material for sealing and receivingthe electrode assembly together with an electrolyte. Particularly, theexterior material is provided with a can having an open end and a capassembly sealed and coupled to the open end of the can.

In general, a gasket is interposed between the can and the cap assemblyto seal a battery. As the gasket of the battery, a layer of a polymerresin such as polypropylene has been used. However, there has been aproblem in that a crimping part of the battery, for example, a trimmingpart in which Fe is exposed, rusts in a high temperature/high humidityatmosphere, so that there has been a demand for solving the problem.

DISCLOSURE OF THE INVENTION Technical Problem

An aspect of the present invention provides a rust inhibitor for agasket, a gasket for a secondary battery including the same, and asecondary battery, the rust inhibitor capable of minimizing thegeneration of rust in a crimping part of a battery in a hightemperature/high humidity atmosphere, and having improved sealingproperties of a gasket.

Technical Solution

According to an aspect of the present invention, there are provided arust inhibitor for a gasket, a gasket for a secondary battery includingthe same, and a secondary battery, the rust inhibitor including ananti-rust material and a polymer resin, wherein the polymer resinincludes at least one of polyethylene or a copolymer which includes 2 ormore derived units selected from the group consisting of anethylene-derived unit, a propylene-derived unit, a butyleneterephthalate-derived unit, an ethylene terephthalate-derived unit, anda methyl acrylate-derived unit, and the anti-rust material includes acompound of Formula 1 below.(A)_(m)-R  [Formula 1]

In Formula 1, R is any one of a saturated or unsaturated aliphatichydrocarbon group having 5 to 20 carbon atoms or an aromatic hydrocarbongroup having 6 to 18 carbon atoms, A is a functional group including, atleast any one of an oxygen atom, a nitrogen atom or a fluorine atom,wherein the hydrogen atom of R is substituted with the functional group,and m is 1 to 3.

According to another aspect of the present invention, there is provideda gasket for a secondary battery including the rust inhibitor for agasket and a base resin.

According to yet another aspect of the present invention, there isprovided a secondary battery including an electrode assembly including apositive electrode and a negative electrode, a can including a topopening, a cap assembly coupled to the can by a crimping part which isformed on an upper outer circumferential surface of the can, wherein aportion of the top opening is bent inward to form the crimping part, andthe gasket for a secondary battery, wherein the gasket for a secondarybattery is interposed between the can and the cap assembly.

Advantageous Effects

A rust inhibitor for a gasket according to an embodiment of the presentinvention includes a specific anti-rust material, and the anti-rustmaterial is evaporated and absorbed to a crimping part, particularly aportion in which Fe is exposed in a top opening of a can, to prevent theFe from directly contacting moisture and oxygen. Accordingly, it ispossible to minimize the generation of rust in a crimping part of abattery in a high temperature/high humidity atmosphere. In addition, thespecific anti-rust material is present in the gasket by being dispersedin a small size at an angstrom (Å) level, and thus, when evaporated,voids generated in the gasket are formed to be small at a negligiblelevel. Therefore, when compared with a case in which a typical anti-rustmaterial present in a gasket by being dispersed in a large size at amicro (y m) level is used, when the rust inhibitor for a gasket of thepresent invention is used, the sealing properties of the gasket may bemaintained.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view illustrating one surface of a secondarybattery having a gasket for a secondary battery including a rustinhibitor for a gasket according to an embodiment of the presentinvention.

FIG. 2 is a cross-sectional view illustrating a secondary battery havinga gasket for a secondary battery including a rust inhibitor for a gasketaccording to an embodiment of the present invention.

FIG. 3 is a cross-sectional view illustrating a secondary battery havinga gasket for a secondary battery including a rust inhibitor for a gasketaccording to another embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail tofacilitate understanding of the present invention.

It will be understood that words or terms used in the specification andclaims shall not be interpreted as having the meaning defined incommonly used dictionaries. It will be further understood that the wordsor terms should be interpreted as having a meaning that is consistentwith their meaning in the context of the relevant art and the technicalidea of the invention, based on the principle that an inventor mayproperly define the meaning of the words or terms to best explain theinvention.

The terminology used herein is for the purpose of describing particularexemplary embodiments only and is not intended to be limiting of thepresent invention. The terms of a singular form may include plural formsunless the context clearly indicates otherwise.

It will be further understood that the terms “include,” “comprise,” or“have” when used in this specification, specify the presence of statedfeatures, numbers, steps, elements, or combinations thereof, but do notpreclude the presence or addition of one or more other features,numbers, steps, elements, or combinations thereof.

A rust inhibitor for a gasket according to an embodiment of the presentinvention includes an anti-rust material and a polymer resin, whereinthe polymer resin includes at least one of polyethylene or a copolymerwhich includes 2 or more derived units selected from the groupconsisting of an ethylene-derived unit, a propylene-derived unit, abutylene terephthalate-derived unit, an ethylene terephthalate-derivedunit, and a methyl acrylate-derived unit, and the anti-rust materialincludes a compound of Formula 1 below.(A)_(m)-R  [Formula 1]

In Formula 1, R is any one of a saturated or unsaturated aliphatichydrocarbon group having 5 to 20 carbon atoms or an aromatic hydrocarbongroup having 6 to 18 carbon atoms, A is a functional group including, atleast any one of an oxygen atom, a nitrogen atom or a fluorine atom,wherein the hydrogen atom of R is substituted with the functional group,and m is 1 to 3.

The rust inhibitor for a gasket is a rust inhibitor included in agasket. Specifically, the gasket is a gasket used in a battery, and morespecifically, the battery may be a secondary battery.

The rust inhibitor for a gasket may be a volatile corrosion inhibitor(VCI). The rust inhibitor for a gasket is evaporated and absorbed to acrimping part, particularly a trimming part in which Fe is exposed in atop opening of a can, to prevent the Fe from directly contactingmoisture and oxygen.

The rust inhibitor for a gasket may be present in the base resin of thegasket by being dispersed in molecular units.

The anti-rust material may include a compound of Formula 1 below.(A)_(m)-R  [Formula 1]

In Formula 1, R is any one of a saturated or unsaturated aliphatichydrocarbon group having 5 to 20 carbon atoms or an aromatic hydrocarbongroup having 6 to 18 carbon atoms, A is a functional group including, atleast any one of an oxygen atom, a nitrogen atom or a fluorine atom,wherein the hydrogen atom of R is substituted with the functional group,and m is 1 to 3.

A, which is a functional group, may be electrically and/or chemicallyabsorbed to the crimping part, particularly a portion in which Fe isexposed in the top opening of the can. That is, when the rust inhibitorfor a gasket is evaporated, the anti-rust material may be disposed inthe portion in which Fe is exposed by the presence of the functionalgroup. In addition, R, except for the functional group of the anti-rustmaterial, may form a hydrophobic layer to prevent the Fe from directingcontacting moisture and oxygen. Accordingly, it is possible to minimizethe generation of rust in the crimping part of a battery under hightemperature/high humidity conditions.

Specifically, the anti-rust material may include at least one ofcompounds of Formula or Formula 1-2 as follows.A₁-R₁  [Formula 1-1]A₁-R₂-A₂  [Formula 1-2]

wherein R₁ is any one selected from the group consisting of a linearalkyl group having 8 to 20 carbon atoms, a branched alkyl group having8-20 carbon atoms, an alkenyl group having 8 to 20 carbon atoms, alkynylhaving 8 to 20 carbon atoms, and an aryl group having 8 to 18 carbonatoms;

R₂ is any one selected from the group consisting of a linear alkylenegroup having 8 to 20 carbon atoms, a branched alkylene group having 8 to20 carbon atoms, an alkenylene group having 8 to 20 carbon atoms,alkynylene having 8 to 20 carbon atoms, and an arylene group having 8 to18 carbon atoms; and

A₁ and A₂ are each independently any one selected from the groupconsisting of a carboxyl group, an amino group, a nitro group, a hydroxygroup, and a fluorine group.

When compounds according to Formula 1-1 or Formula 1-2 are used, A₁and/or A₂ are electrically and/or chemically absorbed to the crimpingpart, particularly a portion in which Fe is exposed in the top openingof the can, and R₁ and/or R₂ may form a hydrophobic layer to prevent theFe from directly contacting moisture and oxygen. Accordingly, it ispossible to minimize the generation of rust in the crimping part of abattery under high temperature/high humidity conditions.

Meanwhile, the gasket for a secondary battery of the present inventionis manufactured by a method of injecting a base resin containingpolybutylene terephthalate and the rust inhibitor at a high temperature.The injection temperature of the polybutylene terephthalate is high,which is about 280° C. However, the structure of the anti-rust materialused in the present invention does not break down even at the injectiontemperature. Accordingly, the anti-rust effect by the anti-rust materialmay be maintained even in a finally manufactured gasket.

More specifically, the anti-rust material may include at least one ofFormula 1-3 or Formula 1-4.

In Formula 1-3 and Formula 1-4, A₁ is the same as A₁ in Formula 1-1 andFormula 1-2, and the p may be 4 to 10 and the q may be 3 to 6.

The anti-rust material of Formula 1-3 or Formula 1-4 may exhibit theanti-rust effect described above, and the structure thereof is notdeformed at an injection temperature of the base resin containingpolybutylene terephthalate. In addition, the deterioration of thesealing properties of the gasket may be minimized for the followingreasons.

Typically, materials such as NaNO₂ and NaNO₃, which are used asanti-rust materials, are present in a resin by being dispersed in alarge unit at a micro (μm) level. Therefore, when the materials areevaporated, large voids are generated in the resin constituting agasket. Accordingly, moisture and oxygen from the outside may penetrateinto a battery through the voids, so that the sealing properties of thegasket may be deteriorated and battery performance may be deteriorated.On the other hand, the anti-rust materials of Formula 1-3 and Formula1-4 are present in the gasket by being dispersed in a small size at anangstrom (Å) level, and thus, when evaporated, voids generated in thegasket are formed to be small at a negligible level. Therefore, thedeterioration of the sealing properties of the gasket may be minimized.

More specifically, the anti-rust material may include at least oneselected from the group consisting of decanoic acid, lauric acid, andmyristic acid. When the anti-rust material is used, the anti-rust effectdescribed above may be exhibited, and the structure thereof is notdeformed at an injection temperature of the base resin containingpolybutylene terephthalate. In addition, the deterioration of thesealing properties of the gasket may be minimized.

The polymer resin is mixed with the anti-rust material, and may serve todisperse the anti-rust material in the base resin. In order tofacilitate the dispersion of the anti-rust material in the base resin,the polymer resin may be different from the base resin.

The polymer resin may include at least one of polyethylene or acopolymer which includes 2 or more derived units selected from the groupconsisting of an ethylene-derived unit, a propylene-derived unit, abutylene terephthalate-derived unit, an ethylene terephthalate-derivedunit, and a methyl acrylate-derived unit. Specifically, the materialsmay be mixed and used. The polymer resin is preferable since the polymerresin is easily mixed with the anti-rust material and also easily mixedwith polybutylene terephthalate. In addition, the polymer resin mayfurther improve the anti-rust properties of the gasket. Specifically,when considering the manufacturing process and the mixing with the baseresin, the polymer resin may be a copolymer including anethylene-derived unit and a methyl acrylate-derived unit.

The weight ratio of the polymer resin to the anti-rust material may be99:1 to 80:20, specifically 99:1 to 85:15, more specifically 99:1 to90:10. When the above range is satisfied, the anti-rust effect may bemore effective.

The rust inhibitor may be in the form of a pellet. Specifically, therust inhibitor may be in the form of a pellet formed by mixing thepolymer resin and the anti-rust material and then subjecting the mixtureto a pelletizing process. As a result, the rust inhibitor may besmoothly dispersed in the base resin. When only an anti-rust materialnot in a pellet form (that is, without a polymer resin) is mixed with abase resin, the anti-rust material is not smoothly dispersed in the baseresin, so that an anti-rust function is rapidly deteriorated.

A gasket for a secondary battery according to another aspect of thepresent invention may include a rust inhibitor for a gasket and a baseresin. Here, the rust inhibitor for a gasket is the same as the rustinhibitor for a gasket of the embodiment described above, and therefore,a description thereof will be omitted.

The base resin may include at least one selected from the groupconsisting of polypropylene, polybutylene terephthalate, andpolyphenylene sulfide. The base resin is suitable for sealing a battery,particularly a battery having a cylindrical shape, and has excellentheat resistance, and thus, is preferable. Specifically, the base resinmay be polybutylene terephthalate. The polybutylene terephthalate hashigh heat resistance and low oxygen and moisture permeability, and thus,is more preferable as the base resin of the gasket.

Furthermore, in the case in which the base resin is polybutyleneterephthalate, when the anti-rust material included in the rustinhibitor for a gasket is used, the following advantages may beobtained. Anti-rust materials, such as NaNO₂ and NaNO₃ which aretypically used as anti-rust materials, weaken a C═O bond included in thepolybutylene terephthalate, thereby deteriorating the polybutyleneterephthalate, so that the physical properties of the gasket aredeteriorated. Specifically, the ductility of the polybutyleneterephthalate is deteriorated, so that cracks are likely to be generatedin the gasket by external force. However, the anti-rust material of thepresent invention may minimize the weakening of the C═O bond included inthe polybutylene terephthalate, or has no effect on the bonding, so thatthe physical properties of the gasket may be maintained. Also, thepolybutylene terephthalate has a high injection temperature of 280° C.,and the structure of the anti-rust material of the present inventioninjected together with the polybutylene terephthalate does not breakdown even at the high temperature, so that the anti-rust properties ofthe anti-rust material may be maintained in a finally manufacturedgasket.

The rust inhibitor may be included in the gasket for a secondary batteryin an amount of 2 wt % to 15 wt %, specifically 2 wt % to 10 wt %, morespecifically 5 wt % to 10 wt %. When the above range is satisfied, theanti-rust properties described above may be more improved.

A secondary battery according to yet another embodiment of the presentinvention includes an electrode assembly including a positive electrodeand a negative electrode, a can including a top opening, a cap assemblycoupled to the can by a crimping part which is formed on an upper outercircumferential surface of the can, wherein a portion of the top openingis bent inward to form the crimping part, and a gasket for a secondarybattery, wherein the gasket for a secondary battery may be interposedbetween the can and the cap assembly. Here, the gasket for a secondarybattery is the same as the gasket for a secondary battery of theembodiment described above, and therefore, a description thereof will beomitted.

The secondary battery may have a cylindrical shape, a square shape, andthe like, and specifically, may have a cylindrical shape. When thesecondary battery has a cylindrical shape, the top opening of the canmay also have a cylindrical shape.

Referring to FIG. 1 and FIG. 2, a secondary battery 100 includes a can20 receiving an electrode assembly 10 together with an electrolyte, acap assembly 30 sealed and coupled to an open end of the can 20, and agasket 40 interposed between the can 20 having a cylindrical shape andthe cap assembly 30. The gasket 40 is the same as the gasket for asecondary battery of the embodiment described above.

The material of the can 20 is not particularly limited. The can 20 maybe formed of at least one among stainless steel, steel, aluminum, or anequivalent thereof. Since the can 20 is required to have conductivity, ametal component is used, and the metal component may be susceptible tocorrosion caused by the contact with moisture from the outside. In thisrespect, the can 20 may include a layer made of Fe and a layer made ofNi for preventing the layer made of Fe from corrosion.

The can 20 may have a cylindrical shape, a square shape, and the like,and specifically, may have a cylindrical shape. The open end of the can20 includes a trimming part 21 a, and in the trimming part 21 a, a Fecomponent of the can 20 may be exposed. Specifically, in a finalproduct, the trimming part 21 a may be seen to be included in a crimpingpart 21.

The cap assembly 30 may include a top cap for sealing the open end ofthe can 20 and a safety vent 36. One surface of the safety vent 36 maybe disposed to come in contact with side surfaces, top surface, andbottom surface of the top cap, and the other surface thereof may be bentand disposed so as to come in contact with an inner surface of thegasket 40. The safety vent 36 may be electrically connected to theelectrode assembly 10. A battery having the cap assembly 30 may providea high output instantaneously when used as a power source of a powertool such as an electric drill, and may be stable against externalphysical impact such as vibration and fall.

Particularly, in the cap assembly 30 having a shape in which the safetyvent 36 is bent to surround the top cap, a contact surface of the safetyvent 36 and the top cap may form one or more connection portions, andthe connection portions are formed by welding and the like. The term“welding” used in the present invention is used as a concept includingnot only a literal meaning of welding such as laser welding, ultrasonicwelding, and resistance welding, but also a fastening method such assoldering. The welding may be performed in an assembly process of thecap assembly 30 itself, or may be performed even when the cap assembly30 is installed in the can 20.

The safety vent 36 serves to shut off current or exhaust gas whenpressure rises inside the battery, and may be preferably made of a metalmaterial. The thickness of the safety vent 36 may vary depending on thematerial and the structure thereof, and is not particularly limited aslong as the safety vent 36 may discharge gas and the like whilerupturing when a predetermined high pressure is generated in thebattery. For example, the thickness of the safety vent 36 may be 0.2-0.6mm.

The thickness of the top cap portion which is in contact with the safetyvent 36 is not particularly limited as long as it is in the rangecapable of protecting various components of the cap assembly 30 frompressure applied from the outside. For example, the thickness of the topcap portion may be 0.3-0.5 mm. If the top cap portion is too thin, it isdifficult to exert mechanical stiffness. On the contrary, if the top capportion is too thick, it is not preferable because the capacity of thebattery may be reduced compared to a battery of the same standard due tothe increase in size and weight.

The gasket 40 may form a cylindrical shape or a square shape in whichboth ends are open, or have a shape which varies depending on the shapeof the top opening of the can 20. Specifically, the gasket 40 may have acylindrical shape. It is preferable that one side end of the gasket 40facing an inner surface of the can 20 has a structure of being bent at apredetermined angle toward a center portion, specifically at a rightangle, to be placed in an open portion of the can 20, that is, thecrimping part 21. The other side end of the gasket 40 is initiallyspread in a straight line facing an axial direction of the gasket 40,and then is bent at a predetermined angle toward the center portionduring the pressing process of the can 20 such that and an innercircumferential surface and an outer circumferential surface are foldedin the state of being in close contact with the top cap of the capassembly 30 and an inner surface of the can 20, respectively.

The crimping part 21 is formed on the top of the can 20 such that thecap assembly 30 may be mounted on the open end of the can 20. Morespecifically, the crimping part 21 is formed by beading a top portion ofthe can 20 so as to form an indentation inward, mounting the gasket 40on the open end, inserting the top cap, a PTC element, and an outercircumferential surface of the safety vent (36) in turn, and thenbending the top portion of the can 20 inward. As a result, a shape ofsurrounding the gasket 40 which is positioned on the inner surface ofthe crimping part 21 is achieved, and crimping and pressing processesare performed to mount the cap assembly 30.

The crimping part 21 is formed in a structure in which an end portion isbent inward such that the cap assembly 30 may be stably mounted on anopen top of the can 20 when the gasket 40 is interposed. A side wall ofthe crimping part 21 is formed so as to be perpendicular to a sidesurface of the battery.

The electrode assembly 10 may include a positive electrode and anegative electrode. Referring to FIG. 2, the electrode assembly 10 hastwo electrode plates 11 having different polarities and having a wideplate shape in the form of a roll, and a separator 12 disposed eitherbetween the electrode plates 11 or disposed on the left or right sidesof one electrode plate 11 so as to insulate the electrode plates 11. Atthis time, the electrode plates 11 spaced apart from each other by theseparator 12 may be a positive electrode and a negative electrode. It ispreferable that the electrode assembly 10 has a structure of being woundin the form of a so-called ‘Jelly Roll.’ The electrode assembly 10 mayalso be in the shape in which a positive electrode plate and a negativeelectrode plate of a predetermined size are laminated having theseparator 12 therebetween.

In the inner space of the can 20, the electrode assembly 10 and anelectrolyte (not shown) are received. The electrolyte is to move lithiumions generated by an electrochemical reaction of the electrode plates 11during charging and discharging of the secondary battery 100. Theelectrolyte may be a non-aqueous organic electrolyte which is a mixtureof a lithium salt and a high-purity organic solvent or a polymer using apolyelectrolyte. However, the kind of the electrolyte is notparticularly limited.

Meanwhile, in the center of the can 20, a center pin (not shown) may beinserted, the center pin for preventing the electrode assembly 10 woundin the form of a jelly roll from being unwound and for serving as amoving path of gas inside the secondary battery 100. An upper portion ofthe can 20, that is, an upper portion of the top of the electrodeassembly 10, is provided with a beading part 24 formed by press-bendingfrom the outside to the inside to prevent upward and downward movementof the electrode assembly 10.

The cylindrical battery 100 according to an embodiment of the presentinvention may further include an auxiliary gasket 42. The auxiliarygasket 42 is a gasket for a current shutoff element 38 and is configuredto surround an outer circumferential surface of the current shutoffelement 38. Particularly, the auxiliary gasket 42 comes in contact withan upper portion and a side portion from the outer circumferentialsurface of the current shutoff element 38 to support the upper portionand the side portion of the current shutoff element 38. In addition, theauxiliary gasket 42 serves such that the current shutoff element 38 andthe safety vent 36 are electrically insulated from each other except fora portion in which a protruding portion of the safety vent 36 and thecurrent shutoff element 38 are in contact with each other.

A battery 100 according to another embodiment of the present inventionis shown in FIG. 3. The same components as the reference numeralsdescribed with reference to FIG. 2 are the same members with the samefunctions. Referring to FIG. 3, a cap assembly 30 may include a top capdisposed to seal an open end of a can 20 and to be in contact with aprotruding portion of a gasket 40, a positive temperature coefficient(PTC) element 34 disposed so as to be in contact the top cap, and asafety vent 36 in which one surface thereof is disposed to be in contactwith the PTC element 34 and a portion of the other surface thereof isdisposed to be in contact with the gasket 40.

The PTC element 34 serves to shut off current by greatly increasingbattery resistance when the temperature inside the battery 100 rises.The thickness of the PTC element 34 may also vary depending on thematerial and the structure thereof, and may be, for example, 0.2 mm to0.4 mm. When the thickness of the PTC element 34 is greater than 0.4 mm,internal resistance increases and the size of the battery is increased,so that the capacity of the battery may be reduced compared to a batteryof the same standard. On the contrary, when the thickness of the PTCelement 34 is less than 0.2 mm, it is difficult to exhibit a desiredcurrent shutoff effect at a high temperature, and the PTC element 34 maybe destroyed even by a weak external impact. Accordingly, the thicknessof the PTC element 34 may be appropriately determined within the abovethickness range in consideration of the above points in combination.

The thickness of a top cap portion which is in contact with the PTCelement 34 is not particularly limited as long as it is in the rangecapable of protecting various components of the cap assembly 30 frompressure applied from the outside. For example, the thickness may be0.3-0.5 mm. If the top cap portion is too thin, it is difficult to exertmechanical stiffness. On the contrary, if the top cap portion is toothick, it is not preferable because the capacity of the battery may bereduced compared to a battery of the same standard due to the increasein size and weight.

A secondary battery including the cap assembly 30 provided with the topcap, the PTC element 34, and the safety vent 36 may be used as a powersource of a cell phone, a notebook computer and the like, which stablysupplies a constant output.

The present invention may provide a battery pack in which a plurality oflithium secondary batteries manufactured by the above embodiment areelectrically connected to each other, and the battery pack may be abattery pack used as a power source for a medium-and-large sized devicein one or more devices selected from the group consisting of a powertool, an electric vehicle including an electric vehicle (EV), a hybridelectric vehicle (HEV), and a plug-in hybrid electric car (PHEV), anelectric truck, an electric commercial vehicle, or a power storagesystem.

Hereinafter, with reference to examples and comparative examples, thepresent invention will be described in more detail. However, thefollowing examples are merely illustrative of the present invention andthe scope of the present invention is not limited thereto.

Example 1: Manufacturing of Battery

(1) Manufacturing of Gasket

A copolymer including an ethylene-derived unit and an acrylicacid-derived unit and lauric acid were mixed in a weight ratio of 95:5at 100° C., and then the mixture was subjected to a pelletizing processto prepare a rust inhibitor in the form of a pellet. 240 g of the rustinhibitor and 3,000 g of polybutylene terephthalate were mixed, and thenthe mixture was injected at a temperature of 280° C. to prepare agasket.

(2) Manufacturing of Battery

A can is composed of an inner layer made of Fe and an Ni layer disposedon both surfaces of the inner layer. After disposing the gasket on a topopening of the can, a cap assembly was placed to be in contact with aninner circumferential surface of the gasket, and then a crimping partwas formed. As a result, a battery including the gasket wasmanufactured.

Example 2: Manufacturing of Battery

(1) Manufacturing of Gasket

A copolymer including an ethylene-derived unit and an acrylicacid-derived unit and lauric acid were mixed in a weight ratio of 95:5at 100° C., and then the mixture was subjected to a pelletizing processto prepare a rust inhibitor in the form of a pellet. 150 g of the rustinhibitor and 3,000 g of polybutylene terephthalate were mixed, and thenthe mixture was injected at a temperature of 280° C. to prepare agasket.

(2) Manufacturing of Battery

A can is composed of an inner layer made of Fe and an Ni layer disposedon both surfaces of the inner layer. After disposing the gasket on a topopening of the can, a cap assembly was placed to be in contact with aninner circumferential surface of the gasket, and then a crimping partwas formed. As a result, a battery including the gasket wasmanufactured.

Example 3: Manufacturing of Battery

(1) Manufacturing of Gasket

A copolymer including an ethylene-derived unit and an acrylicacid-derived unit and decanoic acid were mixed in a weight ratio of 95:5at 100° C., and then the mixture was subjected to a pelletizing processto prepare a rust inhibitor in the form of a pellet. 240 g of the rustinhibitor and 3,000 g of polybutylene terephthalate were mixed, and thenthe mixture was injected at a temperature of 280° C. to prepare agasket.

(2) Manufacturing of Battery

A can is composed of an inner layer made of Fe and an Ni layer disposedon both surfaces of the inner layer. After disposing the gasket on a topopening of the can, a cap assembly was placed to be in contact with aninner circumferential surface of the gasket, and then a crimping partwas formed. As a result, a battery including the gasket wasmanufactured.

Comparative Example 1: Manufacturing of Battery

(1) Manufacturing of Gasket

Polybutylene terephthalate was injected at 280° C. to prepare a gasket.

(2) Manufacturing of Battery

A can is composed of an inner layer made of Fe and an Ni layer disposedon both surfaces of the inner layer. After disposing the gasket on a topopening of the can, a cap assembly was placed to be in contact with aninner circumferential surface of the gasket, and then a crimping partwas formed. As a result, a battery including the gasket wasmanufactured.

Comparative Example 2: Manufacturing of Battery

(1) Manufacturing of Gasket

Polyethylene and NaNO₂ were mixed in a weight ratio of 70:30 at 140° C.,and then the mixture was subjected to a pelletizing process to prepare arust inhibitor in the form of a pellet. 240 g of the rust inhibitor and3,000 g of polybutylene terephthalate were mixed, and then the mixturewas injected at a temperature of 280° C. to prepare a gasket.

(2) Manufacturing of Battery

A can is composed of an inner layer made of Fe and an Ni layer disposedon both surfaces of the inner layer. After disposing the gasket on a topopening of the can, a cap assembly was placed to be in contact with aninner circumferential surface of the gasket, and then a crimping partwas formed. As a result, a battery including the gasket wasmanufactured.

Comparative Example 3: Manufacturing of Battery

(1) Manufacturing of Gasket

12 g of lauric acid and 3,228 g of polybutylene terephthalate weremixed, and then the mixture was injected at a temperature of 280° C. toprepare a gasket.

(2) Manufacturing of Battery

A can is composed of an inner layer made of Fe and an Ni layer disposedon both surfaces of the inner layer. After disposing the gasket on a topopening of the can, a cap assembly was placed to be in contact with aninner circumferential surface of the gasket, and then a crimping partwas formed. As a result, a battery including the gasket wasmanufactured.

Experimental Example 1: Evaluation of Anti-Rust Function

The battery of each of Examples 1 to 3 and Comparative Examples 1 to 3was stored for 2 weeks in the environment of 65° C. and 90% humidity,the anti-rust function thereof was evaluated. The evaluation results areshown in Table 1.

Specifically, based on the region of a trimming part of an upper outercircumferential surface of the can, the ratio of a region in whichcorrosion occurred was represented by %. When the region in whichcorrosion occurred was greater than 50%, 3 points were given. Whengreater than 10% and 50% or less, 2 points were given, and when 10% orless, 1 point was given. When there was no corrosion, 0 point was given.After scoring 30 batteries in the manner described above, the averagevalue of the scores was converted to a score of 10 (averagevalue×(10/3)) to evaluate an anti-rust function with the final scorederived therefrom. Meanwhile, the regions in which corrosion occurredwere confirmed with the naked eye and a microscope.

TABLE 1 Anti-rust function evaluation score Example 1 0.4 Example 2 1.1Example 3 0.5 Comparative 9.7 Example 1 Comparative 1.9 Example 2Comparative 4.9 Example 3

Referring to Table 1, in the case of Examples 1 to 3 in which the rustinhibitor for a gasket of the present invention was used, the anti-rustfunction was confirmed to be much better than Comparative Example 1 inwhich a rust inhibitor was not used and Comparative Example 2 in which atypical rust inhibitor containing NaNO₂ was used. In addition, in thecase of Comparative Example 3, since the rust inhibitor includes only ananti-rust material not a polymer resin, the anti-rust material was notdispersed smoothly, so that the anti-rust function was confirmed to bedeteriorated.

Experimental Example 2: Evaluation of Sealing Function

The battery of each of Examples 1 to 3 and Comparative Examples 1 to 3was stored for 40 days at a temperature of 72° C. in an SOC 100% state.The weight of the battery before and after the storage were measured,and the measurement results are shown in Table 2. The weight change rateof Table 2 was calculated by the following equation.Weight change rate=[(battery weight before storage−battery weight afterstorage)/battery weight before storage]×100

TABLE 2 Battery weight Battery weight before storage after storageWeight change (g) (g) Rate (%) Example 1 48.03 48.02 0.0208 Example 247.97 47.96 0.0208 Example 3 47.94 47.93 0.0208 Comparative 47.90 47.890.0209 Example 1 Comparative 47.97 47.27 1.4592 Example 2 Comparative47.99 47.98 0.0208 Example 3

Referring to Table 2, in the case of Example 1, despite the evaporationof the anti-rust material, voids in the gasket were not large, so thatthe sealing function was confirmed to be effectively maintained as inthe case of Comparative Example 1 in which an anti-rust material was notincluded. Meanwhile, in the case of Comparative Example 2 in which NaNO₂was used as an anti-rust material, a number of large voids were presentin the gasket, so that the sealing function was confirmed to be greatlydeteriorated.

The invention claimed is:
 1. A rust inhibitor for a gasket comprising:an anti-rust material; and a polymer resin, wherein the polymer resinincludes at least one of polyethylene or a copolymer which includes 2 ormore derived units selected from the group consisting of anethylene-derived unit, a propylene-derived unit, a butyleneterephthalate-derived unit, an ethylene terephthalate-derived unit, anda methyl acrylate-derived unit, and the anti-rust material includes acompound of Formula 1 below:(A)_(m)-R  [Formula 1] wherein, in Formula 1, R is any one of asaturated or unsaturated aliphatic hydrocarbon group having 5 to 20carbon atoms or an aromatic hydrocarbon group having 6 to 18 carbonatoms, A is a functional group including at least any one of an oxygenatom, a nitrogen atom or a fluorine atom, wherein a hydrogen atom of Ris substituted with the functional group, and m is 1 to 3, wherein theweight ratio of the polymer resin to the anti-rust material is 99:1 to80:20.
 2. The rust inhibitor of claim 1, wherein the anti-rust materialcomprises at least one of a compound of Formula 1-1 or a compound ofFormula 1-2 below:(A)_(m)-R  [Formula 1]A₁-R₂-A₂  [Formula 1-2] wherein R₁ is any one selected from the groupconsisting of a linear alkyl group having 8 to 20 carbon atoms, abranched alkyl group having 8-20 carbon atoms, an alkenyl group having 8to 20 carbon atoms, alkynyl having 8 to 20 carbon atoms, and an arylgroup having 8 to 18 carbon atoms, R₂ is any one selected from the groupconsisting of a linear alkylene group having 8 to 20 carbon atoms, abranched alkylene group having 8 to 20 carbon atoms, an alkenylene grouphaving 8 to 20 carbon atoms, alkynylene having 8 to 20 carbon atoms, andan arylene group having 8 to 18 carbon atoms, and A₁ and A₂ are eachindependently any one selected from the group consisting of a carboxylgroup, an amino group, a nitro group, a hydroxy group, and a fluorinegroup.
 3. The rust inhibitor of claim 2, wherein the anti-rust materialcomprises at least one of a compound of Formula 1-3 or a compound ofFormula 1-4 below:

wherein, in Formula 1-3 and Formula 1-4, p is 4 to 10, and q is 3 to 6.4. The rust inhibitor of claim 3, wherein the anti-rust materialcomprises at least one selected from the group consisting of decanoicacid, lauric acid, and myristic acid.
 5. A gasket for a secondarybattery, the gasket comprising: a rust inhibitor for a gasket of claim1; and a base resin.
 6. The gasket of claim 5, wherein the rustinhibitor is included in the gasket for a secondary battery in an amountof 2 wt % to 15 wt %.
 7. The gasket of claim 5, wherein the anti-rustmaterial is present in the gasket by being dispersed in a size at anangstrom (Å) level.
 8. The gasket of claim 5, wherein the rust inhibitoris in a form of a pellet.
 9. A secondary battery comprising: anelectrode assembly including a positive electrode and a negativeelectrode; a can including a top opening; a cap assembly coupled to thecan, wherein a portion of the top opening is bent inward to form thecrimping part; and the gasket for a secondary battery of claim 5,wherein the gasket for a secondary battery is interposed between the canand the cap assembly.